Steven Rattner may have helped save the US auto industry, but he was hoodwinked by the horse-trainer former boyfriend of Georgina Bloomberg — who made $1 million in construction kickbacks while hiring contractors to do shoddy, overpriced work on his estate, the financier alleges in a new Manhattan lawsuit.

The filing claims dapper Irish horse trainer Declan Orpen — who dated former Mayor Bloomberg’s equestrienne daughter — “engaged in a pattern of self-dealing and deceit designed to cheat [Rattner] and line his own pockets​.”

Bloomberg’s trainer, Jimmy Doyle, a fellow Irishman, introduced her to Orpen and another Celtic show-jumper named Cian O’Connor in 2008, sparking rumors that she had a fetish for frisky Irishmen.

Car czar Rattner, who led President Obama’s effort to jump start Motor City and is now chairman of Michael Bloomberg’s investment firm Willett Advisors, hired Orpen in 2008 to work at his North Salem country estate, Monomoy Farm.

He “trusted Orpen to oversee Monomoy Farm and handle his various duties on behalf of [Rattner] with honesty, loyalty and diligence” the suit says.

But after Orpen left the estate in 2012, Rattner learned that the groundskeeper had funneled $1 million in construction projects for the farm and Rattner’s residence to a firm called Yeats, while taking an undisclosed $45,000 cut for himself, the suit states. He also failed to tell his employer he had an interest in the company, according to the civil suit.

Orpen, a professional show horse rider, competitor and trainer, also snagged $20,000 in cash for himself and $6,500 for his mother in Ireland from a builder he hired to do a $120,000 addition to Rattner’s horse barn.

But the work didn’t even meet building codes and the former Lehman Brothers banker has spent $700,000 to fix it, he gripes in the filing.

Orpen made $15,000 off a pumped-up $28,000 bid for a stone driveway for Rattner, who also accuses him of “shaking down” a Martha’s Vineyard real estate broker for a piece of commissions she earned helping Rattner find homes for his summer help.

Rattner discovered the swindle when Orpen voluntarily left his job in 2012 after other employees ratted him out, according to the filing.

Orpen did not respond to requests for comment.

He filed the suit “in part to alert others of Orpen’s fraudulent ways and poor character.”

The Brown University grad also wants to snatch back $590,000 in salary, commissions and bonuses he paid to the “faithless servant” over four years.